In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such a hot process gas, often referred to as a flue gas, containing, among other components, carbon dioxide, CO2. The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognised, and have resulted in the development of processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels.
WO 2006/022885 describes one such process for absorbing carbon dioxide from a flue gas. In the process described in WO 2006/022885 the flue gas is first treated by means of conventional air pollution control processes, like particulate collectors, NOx and SO2 control, acid mist capturing device and more. The flue gas then, after the conventional air pollution control processes, has a temperature of about 40-70° C. The flue gas is then cooled down to, preferably, 0-20° C. by means of direct contact cooling, in which the flue gas is cooled by means of cold water. The flue gas is then brought to a CO2 absorber, in which the flue gas is brought into contact with a low temperature ammoniated slurry or solution, which is carbon dioxide lean. The carbon dioxide is absorbed in the ammoniated slurry or solution, and a clean flue gas, containing very small amounts of pollutants and carbon dioxide, leaves the CO2 absorber. The carbon dioxide rich ammoniated slurry or solution is regenerated in a regenerator, in which the carbon dioxide is stripped, at a temperature of about 50-200° C. and under high pressure, to form a concentrated carbon dioxide rich stream.
A problem of the process described in WO 2006/022885 is that the clean flue gas released from the CO2 absorber will contain a substantial amount of ammonia. In WO 2006/022885 it is described that the ammonia concentration of the clean flue gas can be kept as low as possible by means of low flue gas temperature in the CO2 absorber, low NH3/CO2 ratio in the slurry of the CO2 absorber, and a cold water wash of the flue gas before releasing it to the atmosphere. Such water wash can reduce the ammonia concentration in the clean flue gas to the range of about 100-1000 ppm, and preferably to the range of about 100-400 ppm, which is still too high for discharging such a flue gas to the atmosphere.